Touch screens are becoming very popular as an input mechanism for computer systems. Various types of touch screens have emerged, but one format is a Near Field Imaging (NFI) capacitive touch screen. With an NFI capacitive touch screen, an electric field is created on conductive bars within the active portion of the touch screen. When an object, such as a user's finger, comes in close proximity to the active area of the touch screen, it causes a modulation of the electric field, which is sensed by a controller connected to the conductive bars. By analyzing the modulation of the electric field, a location of the contact on the touch screen can be resolved. Typically, the touch screen is surrounded at the edges by a grounded conductive surface (a bezel). To avoid an influence on the electric field due to the conductive bezel, a small region of the touch screen immediately proximate to the bezel is left inactive.
The term “inactive” is intended to mean any mechanism or process by which a region or portion of the touch screen is made to be less susceptible to variations in the electric field than other regions or portions of the touch screen. For example, portions of the underlying sensing circuit may be shielded to avoid recognizing contact or touches in that area. Alternatively, the sensing circuitry may be omitted from a portion of the screen to turn it inactive. Moreover, some combination of eliminating active components together with shielding other active components may be employed.
Some aspects of NFI capacitive touch screens have made them popular for use in difficult environments, such as outdoors applications and applications in the manufacturing industry. For example, NFI capacitive touch screens are effective at discriminating between near field signals and far field signals. Thicker protective layers may be used over the conductive bars while still achieving satisfactory performance. Thicker protective layers enable NFI touch screens to be manufactured that can withstand many of the environmental challenges of these difficult environments. However, certain environmental conditions continue to pose a problem. For instance, NFI capacitive touch screens may be used in environments where the screen is susceptible to conductive liquids like water. These are common in manufacturing and other such applications. A problem occurs if the conductive liquid streams down the touch screen to effectively bridge the inactive region between the active area of the touch screen and the conductive bezel. A similar problem occurs if the conductive liquid streams down at the top of the screen from the conductive bezel over the inactive region onto the active area. If this occurs, errors or false touches are generally registered.
Attempts have been made to address this problem. For instance, the inactive region has sometimes been made of sufficient width that accumulated liquids along the bottom lip of the bezel did not reach the active area of the touch screen. However, this solution did not address the more likely situation that the liquid forms a stream running down the touch screen. Another attempted solution was to try and filter out false touch signals created by streaming liquids. The controller was configured to filter out touch profiles that were highly likely to have been caused by streaming liquids. Although this solution was effective at substantially reducing errors and false touches, it introduced a longer processing delay to register actual touches, which is an undesirable byproduct. Unfortunately, an acceptable solution to this and other similar problems has eluded those skilled in the art.